The present invention relates to tubular bumper beams useful in bumper systems, such as for the front and rear of vehicles. However, it is contemplated that aspects of the present invention are not limited to only front and rear bumper beams, as described below.
Tubular rollformed bumper beams manufactured from ultra-high-strength steel (UHSS) material with high-tensile strengths (such as materials having 140 ksi tensile strength) provide excellent impact characteristics and high strength-to-weight ratios. However, due to the limited elongation associated with UHSS materials, there is a limit to the forming that can be done to the rollformed section before material cracking occurs. This limitation can provide challenges to packaging a rollformed cross section into the required space provided by the original equipment manufacturer (OEM). Specifically, it is difficult to make the cross section small enough and the bends sharp enough to meet OEM space and shape requirements. This problem is aggravated when the beam has a longitudinal curvature/sweep. Further, there is sometimes a need to lower the initial crush strength of the rollformed section in order to control the total impact load and impact load spikes being transferred to the frame rail during impact. Bumper development programs already have long lead times, and it is desirable to provide design solutions that provide high strength while maintaining design flexibility and while also maintaining the ability to provide relatively short lead times as the bumper system is tuned to provide optimal impact/load characteristics for a particular model vehicle.
Nees U.S. Pat. No. 6,643,931 (assignee Shape Corp) discloses a method of manufacturing a one-piece tubular door beam, where non-tubular portions of the beam are annealed so as to create integral end brackets (16) and transition areas (14) with optimal impact characteristics. However, Nees '931 does not teach where or how to apply this technology to bumper beams, nor to tubular reinforcement beams where the entire beam is tubular. Notably, bumper beams such as those used for the front and/or rear of vehicles, have specialized requirements different from door beams. For example, bumper beams must not only withstand substantially higher impact loads, but must also pass a variety of different tests such as 30 degree fixed barrier impacts, flat barrier impacts, 5 mph and higher speed impacts, center post impacts, corner impacts, and pedestrian safety impact testing. Bumper beams must also be shaped to support fascia and other front end components, while permitting air flow to a radiator and an engine compartment. Also, door beams typically have a much smaller cross-sectional shape and typically do not have a tubular section extending a full length of the door beam (due to the space limitations within a door assembly where they are located, and based on OEM's side impact loading requirements which are considerably lower than front end impact requirements).